In 1885, Hermann Ebbinghaus discovered the exponential nature of forgetting. We can roughly describe forgetting with R=e(-t/S) where R is memory retention[?], S is relative strength of memory and t is time. For this reason, strength of memory (S) is the best way to represent the forgetting rate. If you learn a foreign word today, in most cases, you stand only 90% chance to remember it after several days.
The forgetting rate tends to decrease after each repetition. Some strategies for learning aim to reduce forgetting by spaced repetition, under which the learned material is reviewed repeatedly. The spacing between the repetition aims to time the reviews so that the material is reviewed before it would have been forgotten.
wikipedia.org dumped 2003-03-17 with terodump